1. Field of the Invention
The present invention relates to a blade for doctoring of a roll or similar moving surface, sizing or creping of a fibrous material web in a machine for the production and/or finishing of a web, for example of a paper, board or tissue web, the blade including means or devices for the measurement of pressure, force or other operating parameters.
2. Description of the Related Art
The rate of wear of a blade in a paper machine varies significantly. Depending on the blade's position, its working life can vary from hours to days. The degree of wear and condition of the blade thus is a valuable piece of information. If the degree of wear is known, replacements can be predicted and failure can be noticed immediately. If a worn-out or damaged blade is used, the doctoring or creping result will be poor. Also the blade unit or even the surface being doctored can be damaged by a worn doctor blade. There are few effective means or methods for monitoring the condition of the blade while the paper machine is in operation.
The wear of the blade and the doctoring result are particularly affected by the blade load and the blade angle. Usually, a doctor blade is pressed against the surface being doctored by a load imposed on the blade. In known doctor units, the loading devices are calibrated when the paper machine is stopped. The results obtained can thus only be used to give a very rough estimation of the desired blade load. The method can also be applied to determine the blade load during operation, but the method is complicated and the results are inaccurate. These methods also do not provide values for the blade-load over the width of the doctor blade, which would be important information for monitoring the doctoring result and the wear of the doctor blade.
In the state of the art several means or devices for the measurement of operating parameters of a doctor blade in the form of sensors like piezo-electric sensors or strain gauges are described. For example, document DE 10 2008 023966 A1 discloses a pressure setting device having a doctor blade to clean the surface of a roll or cylinder and a measuring device including an analyzing element, which is fitted between the doctor blade and the surface being cleaned. The cylinder is static when the blade pressure is being set. The measuring device may extend over the entire length of the blade. U.S. Patent Application Publication No. 2005/223513 A concerns a calibration device for the pressure of a scraping device blade, which abuts the periphery of a roller or cylinder, comprising a holding blade, a sensor holder mounted thereto, and a pressure sensor, wherein the holding blade, the sensor holder and the pressure sensor are positioned such that the position of the pressure sensor on the periphery of the roller or cylinder corresponds to the position of abutment of the blade. The sensor is a piezo-electrical sensor.
Apart from electrical sensors, also fiber optic sensors are used for monitoring the pressure conditions in a paper machine. Fiber optic sensors generally use a fiber optic waveguide as a sensing element, whereby a strain exerted on the fiber is determined by the impact of the strain on the fiber's optical properties.
U.S. Pat. No. 7,108,766 B shows a doctor unit in a paper machine including a blade carrier having a blade holder fitted to the blade carrier. A doctor blade is mountable in the blade holder to doctor a roll or similar moving surface. The blade holder and/or doctor blade include one or more optical sensors installed inside the construction or on its surface. The sensors are arranged to measure the wear of and/or stress in the blade holder and/or doctor blade.
In conventional fiber optics the strain or bending induced variation in the intensity of light passing the fiber is used as a measurement signal. But since measurement signals obtained by these effects carry no information regarding the location of the signal's origin, it is not possible to determine the position where the optical properties of the fiber have been changed.
A possibility to gain information about the position of the signal's origin is to use more fibers with only one sensor each or to assign a detection unit to each of the sensors. Both possibilities are highly demanding on the technical side and, therefore, expensive in realization.
What is needed in the art is an improved fiber optic sensing system for a doctor blade which avoids the drawbacks of the state of the art and provides a system which allows for determination of a position and strain signals of each sensor.